1. Field of the Invention
This invention is in the field of mechanical vibration damping devices, especially those for damping oscillations by varying the stiffness of structures or connections in buildings during earthquakes and wind storms.
2. Description of Prior Art
Active structural control systems require considerable power to generate control forces that are applied directly to the structure. In semi-active systems the control forces are developed by variation in stiffness of the system, and are not due to direct application of force to the structure; hence, these systems need minimal power to operate.
A variable stiffness device has been developed by Kobori et al. (U.S. Pat. No. 5,147,018). The device consists of a hydraulic cylinder with a double-ended rod and piston and a bypass which allows flow of oil from one side of the piston to the other. The bypass has a valve, which can be closed and opened by a control signal. When the valve is closed the device locks and when the valve is open the device is unlocked. Hence, the device is an on-off device and has only two states, either open or closed. Engaging and disengaging braces using the device varies the stiffness of the building. The main limitation of this device is that the stiffness is switched abruptly from the open to the closed state and vice versa. Patten (U.S. Pat. No. 5,595,372) has developed a similar device that can only vary stiffness abruptly and in a limited fashion.